Exterior Gateway Protocols: EGP, BGP-4, CIDR

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1 Exterior Gateway Protocols: EGP, BGP-4, CIDR Based upon slides of Tim Griffin (AT&T), Ion Stoica (UCB), J. Kurose (U Mass), Noel Chiappa (MIT), Ravi Chandra (Cisco) et Shivkumar Kalyanaraman (Rensselaer Polytechnic Institute) 1 Routage IP Routage par préfixe (Classless routing) une route est composée de la destination l adresse du prochain routeur (next-hop) le masque de réseau permet de déterminer la taille de l espace d adressage concerné (-> préfixe) Choix du préfixe le plus long pour une destination donnée, il faut prendre la route la plus spécifique (le préfixe le plus grand) exemple: adresse destination la table de routage contient /8, /19 and /0 2

2 Routage IP Route par défaut (default route) indique où expédier un paquet si la table de routage ne contient pas une route spécifique c est une configuration courant : la plupart des machines disposent d une (et une seule) route par défaut autre nom : passerelle par défaut (default gateway) 3 Priorité aux routes spécifiques Adresse IP de destination : R3 Tout /8 sauf /16 R1 R2 R /8 -> R /16 -> R /8 -> R /8 -> R /16 Table de routage IP de R2 4

3 Priorité aux routes spécifiques Adresse IP de destination : R3 Tout l/8 sauf 10.1/16 R1 R /8 -> R /16 -> R /8 -> R5.. R & FF est égal à & FF /16 Bingo! Table de routage IP de R2 5 Priorité aux routes spécifiques Adresse IP de destination : R3 Tout /8 sauf /16 R1 R /8 -> R /16 -> R /8 -> R5.. Table de routage IP de R2 6 R & FF.FF.0.0 est égal à & FF.FF /16 Re-Bingo!

4 Priorité aux routes spécifiques Packet: Destination IP address: R3 Tout 10/8 sauf 10.1/16 R1 R2 10/8 -> R3 10.1/16 -> R4 20/8 -> R5.. Table de routage IP de R2 R & FF est égal à & FF /16 Pas de correspondance 7 Priorité aux routes spécifiques Adresse IP de destination : R3 Tout /8 sauf /16 R1 R /8 -> R /16 -> R /8 -> R5.. Table de routage IP de R2 8 R4 10.1/16 Meilleure correspondance, masque réseau de 16 bits

5 Priorité aux routes spécifiques On utilise toujours la route la plus spécifique (celle qui correspond au plus petit volume d adresses IP) La route par défaut est notée /0 ce qui permet d utiliser l algorithme décrit ci-dessus Il y a toujours correspondance. C est la route la moins spécifique. 9 Table d acheminement En anglais : forwarding table Permet de déterminer comment acheminer un paquet dans le routeur Construite à partir de la table de routage Les meilleurs routes sont choisies dans la table de routage Effectue une recherche pour déterminer le prochain saut et l interface de sortie Commute le paquet sur l interface de sortie avec l encapsulation adéquate (ex : PPP, FR, POS) 10

6 Trafic en sortie Paquets qui quittent le réseau Choix de la route (ce que les autres vous envoient) Acceptation d une route (ce que vous acceptez des autres) Politique et configuration (ce que vous faites des annonces des autres) Accords de transit et d échange de trafic 11 Trafic entrant Paquets entrant dans votre réseau Ce trafic dépend de : Ce que vous annoncez à vos voisins Votre adressage et plan d AS La politique mise en place par les voisins (ce qu ils acceptent comme annonces de votre réseau et ce qu ils en font) 12

7 Politique de routage Définition de ce que vous acceptez ou envoyez aux autes connexion économique, partage de charge, etc... Accepter des routes de certains FAI et pas d autres Envoyer des routes à certains FAI et pas à d autres Préferrer les routes d un FAI plutôt que d un autre 13 Pourquoi a-t-on besoin d un EGP? S adapter à un réseau de grande taille hiérarchie limiter la portée des pannes Définir des limites administratives Routage politique contrôler l accessibilité des préfixes (routes) 14

8 History: Default Routes: limits Default routes => partial information Routers/hosts w/ default routes rely on other routers to complete the picture. In general routing signposts should be: Consistent, I.e., if packet is sent off in one direction then another direction should not be more optimal. Complete, I.e., should be able to reach all destinations 15 Core A small set of routers that have consistent & complete information about all destinations. Outlying routers can have partial information provided they point default routes to the core Partial info allows site administrators to make local routing changes independently. CORE S1 S2... Sm 16

9 Peer Backbones Initially NSFNET had only one connection to ARPANET (router in Pittsburg) => only one route between the two. Addition of multiple interconnections => multiple possible routes => need for dynamic routing Single core replaced by a network of peer backbones => more scalable Today there are over 30 backbones! Routing protocol at cores/peers: GGP -> EGP-> BGP-4 17 Autonomous Systems The core + edges were still considered one network => administrative problems like rebooting a router required coordination. Replace this n/w with autonomous systems (AS). Stub AS connect via cores AS = set of routers and networks under the same administration No theoretical limit to the size of the AS All parts within an AS remain connected. If two networks rely on core-as to connect, they don t belong to a single AS 18

10 Autonomous Systems (contd) One router represents the AS to the external world (the core and other AS). This router also collects reachability info ( external routes ) from other AS and diffuses it into its domain. AS is identified by a 16-bit AS number Traffic types: Local = traffic originating or terminating at AS. Transit = non-local traffic AS types: Stub AS => only single connection to one other AS => it carries only local traffic. Multihomed AS: Connected to multiple AS, but does not allow transit traffic Transit AS: carries transit traffic under policy restrictions 19 Exterior Gateway Protocol (EGP) A mechanism that allows non-core routers to learn routes from core (external routes) routers so that they can choose optimal backbone routes A mechanism for non-core routers to inform core routers about hidden networks (internal routes) Autonomous System (AS) has the responsibility of advertising reachability info to other ASs. One+ routers may be designated per AS. Important that reachability info propagates to core routers 20

11 Purpose of EGP you can reach net A via me AS2 AS1 traffic to A R1 EGP R2 A R3 table at R1: dest next hop A R2 R border router internal router Share connectivity information across ASes 21 EGP Operation Neighbor Acquisition: Reliable 2-way handshake Neighbor Reachability: Hellos: j out of m hellos OK => Neighbor UP k out of n hellos NOT OK => Neighbor DOWN Updates/Queries: EGP is an incremental protocol. New info => send updates Each router can query neighbors as well Reachability advertized; metrics ignored Requires a tree topology of ASes to avoid loops (eg: see next slide) 22

12 Why EGP Requires a Tree Structure.. 23 EGP weaknesses EGP does not interpret the distance metrics in routing update messages => cannot be compute shorter of two routes As a result it restricts the topology to a tree structure, with the core as the root Rapid growth => many networks may be temporarily unreachable Only one path to destination => no load sharing Need new protocol => BGP-4 24

13 Today s Big Picture Large ISP Large ISP Stub Dial-Up ISP Small ISP Access Network Stub Stub Large number of diverse networks 25 Autonomous System(AS) Internet is not a single network Collection of networks controlled by different administrations An autonomous system is a network under a single administrative control An AS owns an IP prefix Every AS has a unique AS number ASes need to inter-network themselves to form a single virtual global network Need a common protocol for communication 26

14 Intra-AS and Inter-AS routing a C.b b C d A A.a a b A.c c B.a a B c Gateways: perform inter-as routing amongst themselves b perform intra-as routing with other routers in their AS inter-as, intra-as routing in gateway A.c network layer link layer physical layer 27 Who speaks Inter-AS routing? AS2 AS1 BGP R2 R3 R1 R border router internal router Two types of routers Border router(edge), Internal router(core) Two border routers of different ASes will have a BGP session 28

15 Intra-AS vs Inter-AS An AS is a routing domain Within an AS: Can run a link-state routing protocol Trust other routers Scale of network is relatively small Between ASes: Lack of information about other AS s network (Linkstate not possible) Crossing trust boundaries Link-state protocol will not scale Routing protocol based on route propagation 29 Autonomous Systems (ASes) An autonomous system is an autonomous routing domain that has been assigned an Autonomous System Number (ASN). All parts within an AS remain connected. the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it. RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System 30

16 IP Address Allocation and Assignment: Internet Registries IANA ARIN RIPE APNIC Allocate to National and local registries and ISPs Addresses assigned to customers by ISPs RFC Internet Registry IP Allocation Guidelines RFC Address Allocation for Private Internets RFC An Architecture for IP Address Allocation with CIDR 31 AS Numbers (ASNs) ASNs are 16 bit values through are private Currently over 11,000 in use. Genuity: 1 MIT: 3 Harvard: 11 UC San Diego: 7377 AT&T: 7018, 6341, 5074, UUNET: 701, 702, 284, 12199, Sprint: 1239, 1240, 6211, 6242, ASNs represent units of routing policy 32

17 Voisin (Neighbor) Vocabulaire Routeur avec qui on a une session BGP NLRI/Préfixe NLRI - network layer reachability information Informations concernant l accessibilité (ou pas) d une route (réseau + masque) Router-ID (identifiant de routeur) Adresse IP la plus grande du routeur Route/Path (chemin) Préfixe (NLRI) annoncé par un voisin 33 Vocabulaire (2) Transit - transport de vos données par un réseau tiers, en général moyennant paiement Peering - accord bi-latéral d échange de trafic chacun annonce uniquement ses propres réseaux et ceux de ses clients à son voisin Default - route par défaut, où envoyer un paquet si la table de routage de donne aucune information plus précise 34

18 Nontransit vs. Transit ASes ISP 1 ISP 2 Internet Service providers (ISPs) have transit networks Traffic NEVER flows from ISP 1 through NET A to ISP 2 NET A Nontransit AS might be a corporate or campus network. Could be a content provider 35 Selective Transit NET B NET C NET A DOES NOT provide transit Between NET D and NET B NET A NET D NET A provides transit between NET B and NET C and between NET D and NET C Most transit ASes allow only selective transit key impact of commercialization 36

19 Customers and Providers provider provider customer IP traffic customer Customer pays provider for access to the Internet 37 Customer-Provider Hierarchy provider customer IP traffic 38

20 The Peering Relationship peer provider traffic allowed peer customer traffic NOT allowed Peers provide transit between their respective customers Peers do not provide transit between peers Peers (often) do not exchange $$$ 39 Peering Wars Peer Reduces upstream transit costs Can increase end-to-end performance May be the only way to connect your customers to some part of the Internet ( Tier 1 ) Don t Peer You would rather have customers Peers are usually your competition Peering relationships may require periodic renegotiation Peering struggles are by far the most contentious issues in the ISP world! Peering agreements are often confidential. 40

21 Requirements for Inter-AS Routing Should scale for the size of the global Internet. Focus on reachability, not optimality Use address aggregation techniques to minimize core routing table sizes and associated control traffic At the same time, it should allow flexibility in topological structure (eg: don t restrict to trees etc) Allow policy-based routing between autonomous systems Policy refers to arbitrary preference among a menu of available routes (based upon routes attributes) Fully distributed routing (as opposed to a signaled approach) is the only possibility. Extensible to meet the demands for newer policies. 41 Recall: Distributed Routing Techniques Link State Topology information is flooded within the routing domain Best end-to-end paths are computed locally at each router. Best end-to-end paths determine next-hops. Based on minimizing some notion of distance Works only if policy is shared and uniform Examples: OSPF, IS-IS Vectoring Each router knows little about network topology Only best next-hops are chosen by each router for each destination network. Best end-to-end paths result from composition of all nexthop choices Does not require any notion of distance Does not require uniform policies at all routers Examples: RIP, BGP 42

22 BGP-4 BGP = Border Gateway Protocol Is a Policy-Based routing protocol Is the de facto EGP of today s global Internet Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes : BGP-1 [RFC 1105] Replacement for EGP (1984, RFC 904) 1990 : BGP-2 [RFC 1163] 1991 : BGP-3 [RFC 1267] 1995 : BGP-4 [RFC 1771] Support for Classless Interdomain Routing (CIDR) 43 BGP Operations (Simplified) Establish session on TCP port 179 AS1 Exchange all active routes BGP session AS2 Exchange incremental updates 44 While connection is ALIVE exchange route UPDATE messages

23 Sessions BGP A C AS 100 AS / /24 B D Routeurs BGP appelés peers (voisins) Session entre 2 AS différents = External BGP E AS /24 Connexion TCP/IP ebg- Note: les voisins ebgp doivent être directements raccordés. 45 Sessions BGP A C AS 100 AS / /24 B D Les voisins d un même AS sont appelés des voisins internes (internal peers) E AS /24 Connexion TCP/IP ibgp Note: les voisins ibgp peuvent ne pas être directement connectés. 46

24 Sessions BGP A C AS 100 AS / /24 B D Les voisins BGP s échangent des messages contenant des préfixes (NLRI) E AS /24 Message de mise à jour BGP 47 Configuration de sessions BGP AS 100 Connexion TCP ebgp AS 101 A /30 B.2.1 C D / /24.1 interface Serial 0 ip address interface Serial 0 ip address router bgp 100 network mask neighbor remote-as 101 router bgp 101 network mask neighbor remote-as 100 Les sessions BGP sont établies en utilisant la commande BGP neighbor du routeur Lorsque les numéros d AS sont différents il s agit d une session BGP Externe (ebgp) 48

25 Configuration de sessions BGP A AS 100 AS 101 B Session TCP ibgp / / /24.1 C D interface Serial 1 ip address interface Serial 1 ip address router bgp 101 router bgp 101 network mask network mask neighbor remote-as 101 neighbor remote-as 101 Les sessions BGP sont établies en utilisant la commande BGP neighbor du routeur Numéros d AS différents -> BGP Externe (ebgp) Numéros d AS identiques -> BGP Interne (ibgp) 49 Configuration de sessions BGP AS 100 A B C Connexion TCP/IP ibgp Chaque routeur ibgp doit établir une session avec tous les autres routeurs ibgp du même AS 50

26 Four Types of BGP Messages Open : Establish a peering session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawing previously announced routes. announcement = prefix + attributes values 51 Border Gateway Protocol (BGP) Allows multiple cores and arbitrary topologies of AS interconnection. Uses a path-vector concept which enables loop prevention in complex topologies In AS-level, shortest path may not be preferred for policy, security, cost reasons. Different routers have different preferences (policy) => as packet goes thru network it will encounter different policies Bellman-Ford/Dijkstra don t work! BGP allows attributes for AS and paths which could include policies (policy-based routing). 52

27 BGP (Cont d) When a BGP Speaker A advertises a prefix to its B that it has a path to IP prefix C, B can be certain that A is actively using that AS-path to reach that destination BGP uses TCP between 2 peers (reliability) Exchange entire BGP table first (50K+ routes!) Later exchanges only incremental updates Application (BGP)-level keepalive messages Hold-down timer (at least 3 sec) locally config Interior and exterior peers: need to exchange reachability information among interior peers before updating intra-as forwarding table. 53 Two Types of BGP Neighbor Relationships AS1 External Neighbor (ebgp) in a different Autonomous Systems Internal Neighbor (ibgp) in the same Autonomous System ebgp ibgp is routed (using IGP!) ibgp AS2 54

28 I-BGP and E-BGP I-BGP IGP: Interior Gateway Protocol. Examples: IS-IS, OSPF R2 IGP R3 AS1 E-BGP AS2 A announce B R1 R border router internal router R4 AS3 B R5 55 I-BGP Why is IGP (OSPF, ISIS) not used? In large ASs full route table is very large (100K routes!) Rate of change of routes is frequent Tremendous amount of control traffic Not to mention Dijkstra computation being evoked for any change BGP policy information may be lost I-BGP :Within an AS Same protocol/state machines as EBGP But different rules about advertising prefixes Prefix learned from an I-BGP neighbor cannot be advertised to another I-BGP neighbor to avoid looping => need full IBGP mesh! AS-PATH cannot be used internally. Why? 56

29 IBGP vs EBGP I-BGP nodes: typically ABRs, or other nodes where default routes terminate I-BGP peering sessions between every pair of routers within an AS: full mesh. Physical link IBGP session A C D B AS1 57 ebgp update ibgp Peers: Fully Meshed ibgp updates ibgp is needed to avoid routing loops within an AS Full Mesh => Independent of physical connectivity. Single link may see same update multiple times! ibgp neighbors do not announce routes received via ibgp to other ibgp neighbors. Is ibgp an IGP? NO! Set of neighbor relationships to transfer BGP info 58

30 IBGP Scaling: Route Reflection Add hierarchy to I-BGP Route reflector: A router whose BGP implementation supports the re-advertisement of routes between I-BGP neighbors Route reflector client: A router which depends on route reflector to re-advertise its routes to entire AS and learn routes from the route reflector /16 Route Reflection RR-C1 RR1 RR2 RR3 RR-C4 RR-C2 AS1 RR-C3 EBGP IBGP /24 ER AS2 60

31 Normal IBGP A AS 100 B C 61 Route Reflector: Principle Reflector A B C 62

32 Route Reflector Terminology Route reflector Client Non-client Cluster Cluster ID Normal BGP peer 63 Route Reflector AS 300 A B C AS 100 D AS 200 E F F G 64

33 Route Reflector: Operation Reflector receives path from clients and non-clients Selects best path Best path is from client reflect to non-clients Best path is from non-client reflect to clients Non-meshed clients 65 Route Reflector Divide the backbone into multiple cluster At least one route reflector and few clients per cluster Route reflectors are fully meshed Clients in a cluster could be fully meshed Single IGP to carry next hop and local routes 66

34 Route Reflector: Benefits Solves IBGP mesh problem Packet forwarding is not affected Normal BGP speakers co-exist Multiple reflectors for redundancy Easy migration Multiple levels of route reflectors 67 AS Confederations Divide and conquer: Divides a large AS into sub- ASs Sub-AS AS-1 R1 13 R2 68

35 Inter-domain Routing Without CIDR Service Provider Service Provider Inter-domain Routing With CIDR /16 Global Internet Routing Mesh Global Internet Routing Mesh 69 Longest Prefix Match (Classless) Forwarding Destination = payload Prefix Next Hop Interface OK better / ATM 5/0/ / ATM 5/0/8 even better / Ethernet 0/1/3 best! /23 attached Serial 1/0/7 IP Forwarding Table 70

36 What is Routing Policy Policy refers to arbitrary preference among a menu of available routes (based upon routes attributes) Public description of the relationship between external BGP peers Can also describe internal BGP peer relationship Eg: Who are my BGP peers What routes are Originated by a peer Imported from each peer Exported to each peer Preferred when multiple routes exist What to do if no route exists? 71 Routing Policy Example AS1 originates prefix d AS1 exports d to AS2, AS2 imports AS2 exports d to AS3, AS3 imports AS3 exports d to AS5, AS5 imports 72

37 Routing Policy Example (cont) AS5 also imports d from AS4 Which route does it prefer? Does it matter? Consider case where AS3 = Commercial Internet AS4 = Internet2 73 Import and Export Policies Inbound filtering controls outbound traffic filters route updates received from other peers filtering based on IP prefixes, AS_PATH, community Outbound Filtering controls inbound traffic forwarding a route means others may choose to reach the prefix through you not forwarding a route means others must use another router to reach the prefix Attribute Manipulation Import: LOCAL_PREF (manipulate trust) Export: AS_PATH and MEDs 74

38 Attributes are Used to Select Best Routes /24 pick me! /24 pick me! /24 pick me! /24 pick me! Given multiple routes to the same prefix, a BGP speaker must pick at most one best route 75 (Note: it could reject them all!) BGP Policy Knob: Attributes Value Code Reference ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen] reserved for development We will cover a subset of these attributes Not all attributes need to be present in every announcement From IANA: 76

39 BGP Route Processing Apply Policy = Receive filter routes & BGP tweak Updates attributes Based on Attribute Values Best Routes Apply Policy = filter routes & tweak attributes Transmit BGP Updates Apply Import Policies Best Route Selection Best Route Table Apply Export Policies Install forwarding Entries for best Routes. IP Forwarding Table 77 Import and Export Policies For inbound traffic Filter outbound routes Tweak attributes on outbound routes in the hope of influencing your neighbor s best route selection For outbound traffic Filter inbound routes Tweak attributes on inbound routes to influence best route selection 78 inbound traffic outbound traffic outbound routes inbound routes In general, an AS has more control over outbound traffic

40 Policy Implementation Flow Outgoing Incoming Adj RIB In Main BGP RIB Adj RIB Out IGPs Main RIB/ FIB Static & HW Info 79 Table du routeur BGP BGP RIB Network Next-Hop Path *>i / i *>i / i RIB = Routing Information Base D /24 D /24 D /24 R /16 S /24 Table de routage router bgp 100 network network no auto-summary La commande BGP network peut être utilisée pour remplir la table BGP à partir de la table de routage 80

41 Table du routeur BGP BGP RIB Network Next-Hop Path *> / i * i i s> / i s> / i router bgp 100 network aggregate-address summary-only no auto-summary D /24 D /24 D /24 R /16 S /24 Route Table La commande BGP aggregateaddress permet d installer dans la table BGP une route aggrégée dès que au-moins un sous-réseau est présent 81 Table du routeur BGP Message Processus IN Message BGP RIB Network Next-Hop Path *>i / i *>i / i * > / Processus Out Network Next-Hop Path / Le processus BGP in (entrée) reçoit les messages des voisins place le ou les chemins sélectionnés dans la table BGP le meilleur chemin (best path) est indiqué avec le signe > 82

42 Table du routeur BGP Processus IN BGP RIB Network Next-Hop Path *>i / i *>i / i * > / Processus OUT Message Message Le processus BGP out (sortie) message construit à partir des informations de la table BGP modification du message selon configuration envoi du message aux voisins Network Next-Hop Path / / / Modification du next-hop 83 Commandes BGP de base Configuration router bgp <AS-number> neighbor <ip address> remote-as <as-number> no auto-summary Consultation d informations show ip bgp summary show ip bgp neighbors 84

43 Conceptual Model of BGP Operation RIB : Routing Information Base Adj-RIB-In: Prefixes learned from neighbors. As many Adj-RIB-In as there are peers Loc-RIB: Prefixes selected for local use after analyzing Adj-RIB-Ins. This RIB is advertised internally. Adj-RIB-Out : Stores prefixes advertised to a particular neighbor. As many Adj-RIB-Out as there are neighbors 85 UPDATE message in BGP Primary message between two BGP speakers. Used to advertise/withdraw IP prefixes (NLRI) Path attributes field : unique to BGP Apply to all prefixes specified in NLRI field Optional vs Well-known; Transitive vs Non-transitive 2 octets Withdrawn Routes Length Withdrawn Routes (variable length) Total Path Attributes Length Path Attributes (variable length) Network Layer Reachability Info. (NLRI: variable length) 86

44 ORIGIN: Path Attributes: ORIGIN Describes how a prefix came to BGP at the origin AS Prefixes are learned from a source and injected into BGP: Directly connected interfaces, manually configured static routes, dynamic IGP or EGP Values: IGP (EGP): Prefix learnt from IGP (EGP) INCOMPLETE: Static routes 87 Path Attributes: AS-PATH List of ASs thru which the prefix announcement has passed. AS on path adds ASN to AS-PATH Eg: /16 originates at AS1 and is advertised to AS3 via AS2. Eg: AS-SEQUENCE: Used for loop detection and path selection AS1 (100) /16 AS2 (200) AS3 (15) 88

45 AS-Path AS 200 AS / / / / AS 300 AS /16 AS / / / Traffic Often Follows ASPATH /16 ASPATH = AS /16 AS 2 AS 3 AS 4 IP Packet Dest =

46 But It Might Not AS /16 ASPATH = /25 ASPATH = 5 AS 2 AS 2 filters all subnets with masks longer than /24 AS /16 ASPATH = AS /16 IP Packet Dest = AS /25 From AS 4, it may look like this packet will take path 3 2 1, but it actually takes path Shorter AS-PATH Doesn t Mean Shorter # Hops BGP says that path 4 1 is better than path Duh! AS 4 AS 3 AS 2 AS 1 92

47 Path Attributes: NEXT-HOP Next-hop: node to which packets must be sent for the IP prefixes. May not be same as peer. UPDATE for , NEXT-HOP= BGP Speakers Not a BGP Speaker 93 IBGP and Next Hop AS /16 A B AS 300 C AS / / /

48 Attribut Next-Hop AS /16 A /30 B.2 C / D Network Next-Hop Path / AS /16 E Message BGP AS 100 Prochain routeur pour joindre un réseau /16 Dans une session ebgp c est en général une adresse locale 95 Attribut Next-Hop AS /16 A /30 B AS /16 Message BGP.2 C / Prochain routeur pour joindre un réseau Dans une session ebgp c est en général une adresse locale Le next-hop est mis à jour dans les sessions ebgp 96 D AS /16 Network Next-Hop Path / / E

49 Attribut Next-Hop AS / /30 B.2 A AS /16 C / D AS /16 Network Next-Hop Path / / Le next-hop n est pas modifié dans les sessions ibgp E Message BGP 97 Attribut Next-Hop (suite) Les adresses des next-hops doivent circuler dans l IGP Recherche récursive des routes Permet de concevoir la topologie BGP indépendemment de la topologie physique du réseau En interne les bonnes décisions de routage sont faites par l IGP 98

50 Recursive Lookup If routes (prefix) are learnt thru ibgp, NEXT-HOP is the ibgp router which originated the route. Note: ibgp peer might be several IP-level hops away as determined by the IGP Hence BGP NEXT-HOP is not the same as IP nexthop BGP therefore checks if the NEXT-HOP is reachable through its IGP. If so, it installs the IGP next-hop for the prefix This process is known as recursive lookup the lookup is done in the control-plane (not data-plane) before populating the forwarding table. Example in next slide 99 Join EGP with IGP For Connectivity /16 Next Hop = / Forwarding Table destination next hop / BGP destination next hop / Forwarding Table destination next hop 100 AS AS / / /

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